Assembly - Procedures

Procedures or subroutines are very important in assembly language, as the assembly language programs tend to be large in size. Procedures are identified by a name. Following this name, the body of the procedure is described which performs a well-defined job. End of the procedure is indicated by a return statement.

Syntax

Following is the syntax to define a procedure −

proc_name:
procedure body
...
ret

The procedure is called from another function by using the CALL instruction. The CALL instruction should have the name of the called procedure as an argument as shown below −

CALL proc_name

The called procedure returns the control to the calling procedure by using the RET instruction.

Example

Let us write a very simple procedure named sum that adds the variables stored in the ECX and EDX register and returns the sum in the EAX register −

When the above code is compiled and executed, it produces the following result −

The sum is:
9

Stacks Data Structure

A stack is an array-like data structure in the memory in which data can be stored and removed from a location called the 'top' of the stack. The data that needs to be stored is 'pushed' into the stack and data to be retrieved is 'popped' out from the stack. Stack is a LIFO data structure, i.e., the data stored first is retrieved last.

Assembly language provides two instructions for stack operations: PUSH and POP. These instructions have syntaxes like −

PUSH operand
POP address/register

The memory space reserved in the stack segment is used for implementing stack. The registers SS and ESP (or SP) are used for implementing the stack. The top of the stack, which points to the last data item inserted into the stack is pointed to by the SS:ESP register, where the SS register points to the beginning of the stack segment and the SP (or ESP) gives the offset into the stack segment.

The stack implementation has the following characteristics −

Only words or doublewords could be saved into the stack, not a byte.

The stack grows in the reverse direction, i.e., toward the lower memory address

The top of the stack points to the last item inserted in the stack; it points to the lower byte of the last word inserted.

As we discussed about storing the values of the registers in the stack before using them for some use; it can be done in following way −